1. Field of the Invention
The present invention relates to a low voltage detection circuit which detects a low voltage state of a power supply voltage to output a reset signal. Specifically, the present invention relates to a technique which is effectively used in a low voltage detection circuit which includes a delay to change a reset signal to a cancel state by providing a predetermined delay time when canceled.
2. Description of Related Art
In a system including an IC (semiconductor integrated circuit), since there is a possibility of false operation of the IC if the power supply voltage decreases, measures are taken to stop the operation of the IC by providing a low voltage detection circuit which detects a low voltage state of the power supply voltage and outputs a signal to reset the system.
Conventionally, as shown in FIG. 5, a typical low voltage detection circuit including delay compares a voltage where a power supply voltage VDD is divided by resistor R1 and R2 with a reference voltage Vref with a comparator 13 and when the power supply voltage VDD is a predetermined electric potential or less, a transistor M4 is turned on to lower electric potential of node N1, the result is judged in a comparator CMP to turn on an output transistor M0 and an output voltage Vout is set to a low level (reset state). When the power supply voltage VDD is a predetermined electric potential or more, the transistor M4 is turned off, the output transistor M0 is turned off with the comparator CMP and the output voltage Vout is set to a high level (cancel state). Then, when the output transistor M0 is turned off, output is changed after a predetermined delay time determined by resistor Rd and time constant of the condenser connected to an external terminal CD so that the IC can start operation in a state where the power source voltage is stable.
However, in a low voltage detection circuit including delay as shown in FIG. 5, there are problems such as variation in delay time according to variation in resistor Rd and temperature attributes. In view of the above, in practical use, a low voltage detection circuit including delay is provided with a constant current circuit including a constant current source and a current mirror circuit which turns back the electric current of the constant current source and a delay time where influence from variation of elements and temperature attributes is low can be set by charging a condenser connected to an external terminal at the constant current circuit (for example, see Japanese Unexamined Patent Application Publication No. H09-116401).
According to the low voltage detection circuit using a current mirror circuit as proposed in the invention of Japanese Unexamined Patent Application Publication No. H09-116401, since an electric current is constantly flown in the constant current source of the current mirror circuit, there is a problem that the consumed electric current increases. Therefore, there may be a technique as shown in FIG. 6, where a switch SW1 is provided in series with a constant current source CI2 and the switch SW1 is turned on and off with output of a comparator 13 to suppress consumption of electric current. The structure in FIG. 6 uses a MOSFET as the transistor composing the circuit and does not include a manual reset function, but the basic structure is similar to that of Japanese Unexamined Patent Application Publication No. H09-116401.
According to the low voltage detection circuit shown in FIG. 6, while the power supply voltage VDD is a predetermined electric potential or less, the switch SW1 is turned off and it is possible to not flow electric current in the constant current source. However, when the power supply voltage VDD is a predetermined electric potential or more and the output transistor M0 is turned on after a predetermined delay time, the switch SW1 continues to be turned on. Therefore, an electric current flows to the constant current source C12 for a relatively long period of time, and there is a problem that it is not possible to sufficiently suppress the consumption of electric current.